The field of this invention is that of tools used for the cleaning of pipelines, especially the long extended reach pipelines in offshore areas. As hot production crude is produced from the reservoirs below the ocean floor up to the wellhead equipment at the ocean floor and then through pipelines along the ocean floor, it is cooled by the relatively cool temperature of the ocean water. In deepwater, the temperature can be as cold as 35 degrees Fahrenheit.
A characteristic common to a majority of the oil produced is that there is a paraffin component to the oil which will deposit on the walls of the pipeline and become a solid at temperatures well above the 35 degrees Fahrenheit. In fact, some of the paraffins become solid at temperatures above 100 degrees Fahrenheit, and so can be deposited or plated on the internal diameters of the pipelines at any expected ambient temperature. The process is similar to discussions of blocking of the arteries of a human being, with a thicker coating building up with time. Some pipelines have become so plugged that more than 90% of the flow area is blocked with the waxes or paraffins.
Typically, the wall becomes layered with paraffin as the temperature of the oil goes below the solidification temperature of the particular paraffins in the produced fluids. The paraffins act as a sort of insulation to the flowing fluids in the pipeline, allowing it to maintain a higher temperature for a greater distance. The effect of this is to extend the distance along the pipeline which the paraffin is plating onto the internal diameter of the pipeline.
A common cure for the paraffin plating out on the internal diameter of the pipeline is to insert a pig into the flow stream and let the pig remove some of the paraffin. A pig is typically a cylindrical or spherical tool which will brush against the internal diameter of the pipeline in hopes of removing the deposited paraffins. In pipelines with a high incidence of deposited paraffins, a regular maintenance of pigs is normally prescribed as a preventative to pipeline blockage.
One problem with the pigs is that the deposited paraffins are relatively soft and contain a lot of oil. To some extent, the pigs actually compress the paraffins against the wall and squeeze the oil out, leaving a harder and stronger paraffin remaining.
A second problem is that when the paraffin layer on the internal diameter of the pipe is too thick, sloughing off may occur. If the paraffin starts to separate from the wall and continues, the pig begins to literally plow a block of paraffin ahead of itself. This will continue driving more and more paraffin off the wall of the pipeline until the pressure of the pipeline will no longer be able to move the mass. At that time you have full pipeline blockage, which cannot be moved by pressure from either end.
At that time the plug of paraffin must be removed by chemicals. Characteristically, the way chemicals are deployed to the location of the blockage is to use a string of coiled pipe or coiled tubing which is unreeled into the pipeline to provide a circulation path for the circulation of chemicals. As the end of the coiled tubing pipe reaches the location of the blockage, the chemicals are circulated either out the coiled tubing and back through the annulus outside of the coiled tubing and inside the pipeline, or the flow will be in the opposite direction.